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Clock signal usually originates from a crystal.
- Pins EXTAL and XTAL (33 and 34, respectively) are used for the external clock circuit.
- ECLK (pin 29)
Resets and Interrupts
- RESET (pin 32)
- IRQ, XIRQ (pins 37, 38)
Power and Ground
- VDD, VSS (internal) (pins 10 & 47, pins 11 & 48)
- VDDX, VSSX (external) (pins 59, 60)
All CPU circuitry operates according to the system clock. Some CPU circuits work according to the rising edge of the clock and some on the falling edge of the clock. The ideal clock is a square wave with specified duty cycle. The required duty cycles (100% * time of high in a period/ period) for the clock input differ from different microprocessors. Each CPU specifies a maximum clock speed and minimum clock speed. The source of clock can be from a quartz crystal, or an RC circuits, or an external circuit. Most CPUs are driven by quartz crystal. Each CPU has its own suggested ways to connect a clock source to it. The frequency of the crystal should be chosen as an integer multiple of the desired system clock frequency (depends on the given CPU).